Mentally Subtracting Two-Digit Numbers
How to mentally subtract two two-digit numbers.
- Subtract 35 from 84.
- First subtract the two tens' place digits (8 - 3 = 5)
- Notice that the bottom ones' digit is larger than the top ones' digit. Decrease the answer for the tens' place by one (5 - 1 = 4) and increase the top ones' place value by 10 (4 + 10 = 14).
- Next subtract the two ones' place values (14 - 5 = 9)
- Combine the tens' and ones' place differences to make 49.
- Subtract 62 from 94.
- First subtract the two tens' place digits (9 - 6 = 3)
- Next subtract the two ones' place digits (4 - 2 = 2)
- Combine the tens' and ones' place differences to give the answer of 32
Subtracting 2-digit numbers mentally
There are several strategies or methods to subtract two 2-digit numbers mentally.
| Strategy 1: Subtract in two parts | ||||||||||||||||||
57 − 25 = ? Break the 25 into tens and ones. First subtract 20, then subtract 5.
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1. Subtract mentally by breaking the second number into tens and ones.
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2. How can you compare these expressions without actually subtracting? Write < or > .
60 − 28 | 60 − 25 | 90 − 25 | 90 | |||
70 − 24 | 70 − 36 | 97 − 32 | 90 − 32 |
| Strategy 2: Add up to find the difference. | ||||||||||
| Subtraction gives you the difference between two numbers. To find the difference, you can start at the smaller number, and add up till you get to the bigger number. When adding up, first complete the ten, then add whole tens, then ones again.
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3. Add up to find the difference.
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| 56 − 28 = 24 − 55 = | 72 − 18 = 82 − 46 = | 74 − 55 = 63 − 34 = |
4. Find missing addends. The same method works here. Think: first add up to whole ten, then see how much more you need.
a. 13 + __ = 30 37 + __ = 70 28 + __ = 90 54 + __ = 80 | d. 36 + __ = 60 36 + __ = 64 65 + __ = 80 65 + __ = 83 |
| Strategy 3: Subtract an easy number that is close, then correct the answer. | |||||
This strategy works well if the number you're subtracting is a little less than a whole ten. You instead subtract the whole tens, and then add back however much the "error" was.
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5. Subtract mentally using the above strategy.
| 34 − 18 = 42 − 29 = 76 − 59 = | 65 − 27 = 55 − 38 = 94 − 48 = |
6. Subtract mentally, using any of the methods - or one of your own!
| 66 − 38 = 93 − 57 = 27 − 41 = 62 − 35 = 88 − 49 = | 55 − 46 = 74 − 28 = 48 − 13 = 74 − 53 = 91 − 59 = |
Practice: Mentally Subtracting Two-Digit Numbers
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